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Even professional skeptics have been converted. "When I was younger, I believed that 100% of sex differences were due to the environment," says Jerre Levy, professor of psychology at the University of Chicago. Her own toddler toppled that utopian notion. "My daughter was 15 months old, and I had just dressed her in her teeny little nightie. Some guests arrived, and she came into the room, knowing full well that she looked adorable. She came in with this saucy little walk, cocking her head, blinking her eyes, especially at the men. You never saw such flirtation in your life." After 20 years spent studying the brain, Levy is convinced: "I'm sure there are biologically based differences in our behavior."
Now that it is O.K. to admit the possibility, the search for sexual differences has expanded into nearly every branch of the life sciences. Anthropologists have debunked Margaret Mead's work on the extreme variability of gender roles in New Guinea. Psychologists are untangling the complex interplay between hormones and aggression. But the most provocative, if as yet inconclusive, discoveries of all stem from the pioneering exploration of a tiny 3-lb. universe: the human brain. In fact, some researchers predict that the confirmation of innate differences in behavior could lead to an unprecedented understanding of the mind.
Some of the findings seem merely curious. For example, more men than women are lefthanded, reflecting the dominance of the brain's right hemisphere. By contrast, more women listen equally with both ears while men favor the right one.
Other revelations are bound to provoke more controversy. Psychology tests, for instance, consistently support the notion that men and women perceive the world in subtly different ways. Males excel at rotating three-dimensional objects in their head. Females prove better at reading emotions of people in photographs. A growing number of scientists believe the discrepancies reflect functional differences in the brains of men and women. If true, then some misunderstandings between the sexes may have more to do with crossed wiring than cross-purposes.
Most of the gender differences that have been uncovered so far are, statistically speaking, quite small. "Even the largest differences in cognitive function are not as large as the difference in male and female height," Hines notes. "You still see a lot of overlap." Otherwise, women could never read maps and men would always be lefthanded. That kind of flexibility within the sexes reveals just how complex a puzzle gender actually is, requiring pieces from biology, sociology and culture.